The present invention relates to a novel method for the filtration of liquid media, including but not limited to surface water, and to plants for implementing the method.
One method which is conventionally used for treating surface water is slow biological filtration, which is a reproduction of the natural process of water self-purification. Referring to FIG. 1A, this technique is performed by causing a stream of water to be treated to percolate through a bed of filter material at velocities of less than 10 m/day. The operation of such a plant requires a filter material (a), the initial depth of which is about one meter, and a supernatant water height (b) of 1.5 to 2 meters. Consequently, the height of such a construction is between 2.5 and 3 meters. A drainage system (c) collects the filtered water at the bottom of the plant.
In such a plant, a stream of suspended matter from the raw water entering the filter material (a) spreads uniformly over the surface of the filter material (a), causing uniform clogging of the entire filtering surface and resulting in a rise in the supernatant water height (b) so as to maintain a constant output of treated water. After the water height (b) has reached a maximum level, it is necessary to intervene so as to carry out an unclogging operation.
Such unclogging takes place in three main phases, including drainage of the supernatant water, drawing-off of the first four centimeters of clogged filter material, and re-filling of the filter with water to be treated. These three operations are burdensome in their implementation, and take quite a long time to perform, potentially more than a week. It is therefore necessary to provide duplicate plants so as to maintain the production of suitable water.
Conventional slow biological filtration therefore requires the raw water to be of a quality of less than 10 NTU so as to limit the number of unclogging operations required over time. Moreover, the drawing-off of filter material during successive unclogging operations results in a gradual reduction in the height of the filter material. After a limiting height of 40 to 50 cm is reached, it is necessary to restock the filter material, presenting a limiting factor in the management of the plant.
It is an object of the present invention to overcome the constraints of such prior filtration systems.
To achieve this it has been found that, under certain operating conditions, proceeding contrary to the conventional teachings can lead to a new and unexpected phenomenon.
The conventional recycling of an outgoing stream is performed by reinjecting the outgoing stream into the incoming stream, upstream of a plant, which results in a dilution of the incoming stream. However, such conventional recycling does not lead to any improvement.
In accordance with the present invention, the outgoing stream is reinjected onto the surface of the filter medium, and the reinjected stream is caused to flow counter to the incoming stream while maintaining a small height of supernatant water so that there is no dilution of the streams. As a result, there is a localized deposition of matter on the surface of the filter material, initially near the inlet of the plant and then progressively expanding toward the outlet, until the entire surface of the filter material is covered.
The present invention is directed to a method for the filtration of liquid media, including but not limited to surface water, in which the liquid to be filtered, having a flow rate (Q), is brought into contact with the surface of a filter medium and is then passed through the filter medium. A supernatant liquid layer, having a small thickness (e), is created on the surface of the filter medium, the high level of which is suitably controlled. A hydraulic path, having a length (L), is organized in this thin supernatant layer, and a stream of liquid to be treated, having a flow rate (Q), is brought to the inlet of the hydraulic path. The liquid that has passed through the filter medium after n cycles, having a flow rate (nQ), is recovered by reinjecting it into the supernatant layer at the outlet of the hydraulic path. Thus, in one section of the path, a meeting of two opposed streams is produced. The treated liquid, having a flow rate (Q), is drawn off after n cycles. The ratio e/L is small, and is preferably less than 0.1.
As alternatives, the high level of the supernatant layer having the thickness (e) is determined by the position of the outlet of the treated liquid having the flow rate (Q), or the thickness (e) is limited by choosing a filter medium in tube form, having a small inside diameter.
A plant for implementing the method of the present invention is comprised of a filter medium, onto the surface of which is brought a liquid to be treated, having flow rate (Q), so that the liquid to be treated is passed through the filter medium. The plant comprises means for determining a supernatant liquid layer of a small thickness (e) on the surface of the filter medium, means for determining a hydraulic path of a length (L) in the resulting thin supernatant layer, and an inlet for the liquid to be treated, located at one end of the hydraulic path. A recycling loop takes the liquid from the exit from the filter medium to an injection point located at the other end of the hydraulic path, where the outlet for the filtered liquid discharged from the plant is also located.
The method of the present invention overcomes the drawbacks of prior methods, and the plants which implement the method of the present invention are of a simple design. The main advantages include the following. The height of such constructions is considerably reduced (height reduced by a factor of 6). It is possible to treat high levels of suspended matter, for example, surface water and karstic water with a turbidity greater than 300 NTU. Treatment quality can be maintained despite a large variation in the quality of the raw water. The addition of a chemical reactant is not required. The clogged material on the surface is organized and limited without penetration (i.e., piling-up of the suspended matter in the first part of the filter). Clogging only occurs on the surface. There is direct access to the clogged layer from above the filter. Unclogging is easily performed using a tool of the scraper or water-jet type. There is no loss and no draw-off of filter material, hence perpetuity of the plant. Intense biological activity is maintained (since the filter material is highly seeded over the first few centimeters and is not drawn off after each washing of the clogged layer). There is very low energy consumption (less than 0.4 kW per m3 of water produced).
In one embodiment of the plant, the filter medium is sand and the thickness (e) (the high level of the supernatant layer) is determined by the outlet level of the filtered liquid discharged from the plant. In another embodiment, the filter medium is an array of filter tubes having membrane walls through which the liquid passes, and the thickness (e) is limited by the diameter of the tubes.